Bacteriophage MB78 is a virulent phage ofSalmonella typhimurium. The viral DNA is 42 kb in size and seems to be circularly permuted. We show that viral DNA replication is through concatemeric DNA formation which is subsequently converted into full length DNA through headful packaging. A restriction map of MB78 DNA for six restriction endonucleases e.g.BgIII,PvuII, ECORI, ClaI, SalI and SmaI has been constructed. The yield of certain fragments in less than molar amount is explained in terms of permutation and the headful mechanism of packaging. The packaging site (pac site) has been suggested.